In recent researches, how to increase luminous efficiency of a light emitting diode is always the key point. When a light emitting diode is turned on, light beams from the active layer scatter to all directions. Some are efficiently utilized; some are absorbed by the substrate. A good illustration is shown in FIG. 1. A common light emitting diode 10 is composed of a sapphire substrate 102, a LED chip 104 and an encapsulating body 106. The LED chip 104 is epitaxially grown on the sapphire substrate 102 and generates light beams. The LED chip 104 and the sapphire substrate 102 are packaged in and protected by the encapsulating body 106. All arrows represent light beams emitted from the LED chip 104. It is obvious that the light beams scatter omnidirectionally. The solid arrows indicate light beams directly emitting out of the light emitting diode 10 which provides efficient lighting. Meanwhile, the dotted arrows and dashed arrows indicate light beams emitting towards the sapphire substrate 102.
The dotted arrows represent light beams enclosed in the sapphire substrate 102 which does not contribute to lighting of the light emitting diode 10 due to absorption of the sapphire substrate 102 mainly caused by total internal reflection. On the other hand, the dashed arrows show light beams which have a chance to be released from the sapphire substrate 102 without being absorbed. A design to improve the sapphire substrate 102 so that more entrapped light beams can be utilized is desired.
U.S. Pat. No. 6,791,119 shows an improved substrate structure for the purpose above. Please see FIG. 2. Light emitting diodes include a substrate which has first and second opposing faces and is transparent to optical radiation in a predetermined wavelength range. The substrate has a patterned cross-section. A number of pedestals extend into the substrate from the first face towards the second face. A diode region on the second face is configured to emit light in the predetermined wavelength range into the substrate upon application of voltage across the diode region. A mounting support on the diode region, opposite to the substrate, is configured to support the diode region. Therefore, the light emitted from the diode region into the substrate is emitted from the first face upon application of voltage across the diode region. The first face of the substrate may include many grooves that define many triangular pedestals in the substrate. The grooves may include tapered sidewalls and/or a beveled floor. The first face of the substrate also may include an array of via holes. The via holes may include tapered sidewalls and/or a floor.
The '119 patent points out one feature to enhance luminous efficiency: tapered slopes formed due to different sizes of the first and second opposing faces. However, the aforementioned design of '119 patent is mainly ideal for a light emitting diode which is not enclosed by a phosphor. Due to its structure, the light emitting diode of the prior art is not easy to manufacture.
In order to overcome those defects mentioned above and provide an easy way to enhance luminous efficiency of a light emitting diode, the present invention uses an enlarged contact area of light emitting diode substrate for contacting more phosphor. Luminous efficiency is enhanced as well. The present invention is also advantageous to carry out easily.